The present invention relates generally to loudspeakers and, more particularly, to loudspeakers which can be self-cooled through the natural vibratory motion of the loudspeaker diaphragm during normal operation.
Loudspeakers, or speakers, are well known and are commonly used in a wide variety of applications, such as in home theater stereo systems.
Loudspeakers typically comprise a coil of wire, typically referred to as a voice coil, which is suspended between the pole pieces of a permanent magnet. In operation, an alternating current is passed through the voice coil which produces a changing magnetic field around the voice coil. The changing magnetic field around the voice coil interacts with the magnetic field produced by the permanent magnet to produce reciprocal forces on the voice coil. The voice coil is disposed within the speaker so that it can oscillate in accordance with the reciprocal forces. The voice coil is attached to a cone shaped diaphragm which vibrates in response to the forces applied to the voice coil. The vibration of the diaphragm produces sound waves in the air.
Voice coils are constructed of a conductive material. As a consequence, when an electrical signal is passed through the voice coil, the coil will conduct heat. Because the voice coil is typically suspended between the pole pieces of the permanent magnet, which is often a relatively small enclosed volume, it has been found that, in operation, dissipated power in the voice coil leads to significant temperature rise, particularly in high-powered loudspeakers.
Significant temperature rise in the voice coil creates numerous disadvantages.
As a first disadvantage, it has been found that significant temperature rise can increase the resistance of the voice coil. This, in turn, results in a substantial portion of the electrical input power of the loudspeaker to be converted into heat rather than into acoustical energy, thereby limiting the level of performance of the loudspeaker, which is undesirable. In particular, it has been found that increased resistance of the voice coil in the loudspeaker can lead to non-linear loudness compression effects at high sound levels.
As a second disadvantage, it has been found that significant temperature rise can melt bonding materials in the voice coil. This can result in permanent structural damage to the loudspeaker.
As a third disadvantage, it has been found that significant temperature rise can burn out the voice coil. This can result in permanent structural damage to the loudspeaker.
As a consequence, numerous attempts have been made in the art to prevent significant temperature rise in the voice coil.
It is well known in the art to utilize additional components to prevent significant temperature rise in the voice coil. For example, a metallic voice-coil bobbin is often used to conduct heat away from the region of the voice coil. As another example, the voice coil is often coated with a low viscosity fluid to transfer heat produced by the voice coil into the magnetic structure from which it can more easily radiate into the surroundings. As yet another example, heat radiating fins are often mounted on the permanent magnet to improve secondary cooling.
It is also well known in the art to use cooling fans to prevent significant temperature rise in the voice coil. For example, in U.S. Pat. No. 4,757,547 to T. J. Danley, there is disclosed an air cooled speaker in which an electrical blower is used to pass cooling air through a loudspeaker driver. The blower is connected in parallel to the leads between the amplifier and speaker such that the blower speed and cooling increases with increased power consumption and heat generation by the driver.
The use of additional components to prevent significant temperature rise in the voice coil introduces numerous drawbacks. In particular, the use of additional components significantly increases the complexity of the speaker and consequently increases the overall cost of the speaker.
Accordingly, it is well known in the art for loudspeakers to utilize venting techniques to prevent significant temperature rise in the voice coil. Specifically, it is well known in the art for loudspeakers to include ventilating paths, or openings, in the permanent magnetic or in the loudspeaker diaphragm through which cooler air is permitted to pass through the loudspeaker and thereby lower the temperature of the voice coil. Loudspeakers which utilize venting techniques have significant advantages. In particular, such a loudspeaker only requires a change in the shape of the magnetic component or diaphragm and therefore requires no additional components, thereby minimizing complexity and cost.
It is known in the art for loudspeakers to utilize the self-pumping action of the loudspeaker diaphragm during operation to create a flow of air through ventilating paths which, in turn, lower the temperature of the voice coil. For example, in U.S. Pat. No. 5,042,072 to D. J. Button, there is disclosed a self-cooled electrodynamic loudspeaker wherein the magnetic structure or pole piece has channels whereby cool air may be introduced and hot air may be exhausted to cool a voice coil by movement of the speaker diaphragm. This self-cooling results in greater power handling and output of the speaker.
In U.S. Pat. No. 5,357,586 to D. D. Nordschow et al, there is disclosed a flow-through air-cooled loudspeaker system. The loudspeaker and enclosure are provided with aerodynamically-shaped passages providing low-pressure regions for inducing flows of air into and about the driver motor of the loudspeaker in response to vibratory movement of the speaker cone. An aerodynamically-shaped body is disposed within the pole piece to define a venturi passage for exchange of air between an interior chamber defined by a coil former and the back of the speaker. Aerodynamically-shaped openings are provided through the pole piece for inducing flow of air about the voice coil in the voice coil gap between the pole piece and permanent magnet. The speaker frame support is provided with aerodynamically-shaped openings to induce air flow into the interior chamber. In this manner, low-pressure regions established by the aero-dynamic shapes induce flow of cooling air about the voice coil and pole piece in response to vibratory movement of the cone. Aerodynamic shapes are disposed in the intake and exhaust vents of the speaker enclosure to exchange air between the enclosure and atmosphere in response to vibratory movement of the speaker.
Loudspeakers which utilize venting techniques typically experience at least one of the following drawbacks.
As a first drawback, it has been found that loudspeakers which utilize venting techniques are experience difficulty in drawing in cooler air and in passing out warmer air. Specifically, it has been found that the same warm air particles which are pushed away from the voice coil during one half of the self-pumping cycle of the loudspeaker are pulled back in towards the voice coil during the second half of the self-pumping cycle. After a period of use, a state of near equilibrium gets established and produces an oscillating column of air within the permanent magnet which has the same temperature as the voice coil, thereby preventing the air from significantly lowering the temperature of the voice coil.
As a second drawback, it has been found that loudspeakers which utilize venting techniques tend to draw stray particles into the proximity of the voice coil. In particular, it has been found that stray magnetized particles are often drawn to the voice coil. As such, it has been found that the stray magnetized particles can get trapped along the voice coil due to the high flux density magnetic field of the loudspeaker. This can cause stray magnetized particles to accumulate along the voice coil to the point that significant mechanical noise is introduced and to the point that movement of the diaphragm is interfered therewith.